1. Field of the Invention
The present invention relates to an antenna device in which a strip-shaped conductor is wound in a spiral around a substrate formed of a dielectric body or a magnetic body so as to be tuned to a desired frequency, and more particularly, to a wideband antenna device suitable for receiving an ultrahigh frequency (UHF) band for television broadcasting and the like.
2. Description of the Related Art
FIG. 4 is a perspective view of a known antenna device 20 in which capacitors are spread over a conductor wound around a magnetic body. FIG. 5 is an equivalent circuit diagram of the antenna device 20 (for example, see JP A 51-83755(Pages 2-3, FIG. 3)). In the known antenna device 20 shown in FIG. 4, a spiral conductor 22 is wound around outer circumferential surfaces of a ferrite core 21. Ends of the spiral conductor 22 serve as connection terminals 23 and 24. The spiral conductor 22 is formed by a plurality of split conductor portions 22a connected in series with each other. The adjacent split conductor portions 22a are connected to each other with capacitors 25 therebetween. That is, as shown in the equivalent circuit diagram of FIG. 5, the antenna device 20 forms a closed loop circuit in which the capacitors 25 are spread over a line of the spiral conductor 22, and the antenna device 20 is capable of being tuned to a predetermined frequency by supplying a radio-frequency signal to the connection terminals 23 and 24.
However, in the known antenna device 20, excellent receiver sensitivity is achieved only in a limited frequency band. Thus, for example, a UHF band for television broadcasting cannot be received over a range from a lower frequency side (470 MHz to 620 MHz) to a higher frequency side (620 MHz to 770 MHz).